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Meet Interstitium, the new ‘organ’

The problem lies in the definition of an organ

With our current prowess in the world of technology, you cannot blame humanity for being surprised at the discovery of a new organ. It’s not like we haven’t cut open enough human bodies to figure everything out yet, right? Turns out, we’ve been doing it wrong all along with respect to one particular organ.

Scientists have found a network of fluid-filled spaces between tissues that they hadn’t seen before. According to a report published in journal Scientific Reports, these fluid-filled spaces have been discovered in connective tissue all over the body – under the skin, lining the digestive tract, urinary system and lungs. So, meet the human interstitium.

Interstitium in a diagram

What is it?

The human interstitium isn’t particularly new. We’ve been aware of it for ages. To give you an idea, imagine it as what fills the space between cells in the human body – that is what we’ve known about it so far. Turns out, it is more like a matrix of collagen bundles interspersed with fluid than the densely-packed stacks of connective tissue it appears to be in fixed slides. So, rather than a wall surrounding your cells, it is more like a highway that goes all around them.

Previously, tissue samples had to be explored through fixed slides, which would involve getting a thin cut of the sample tissue treated chemically first. This would drain out the fluid that made up a major portion of the interstitium. But when coauthors and physicians Petros Benias and David Carr-Locke used a recent technique to assess patients’ bile ducts for cancer spread, they saw something that hasn’t been seen before.

A flaw in the normal tissue sample creation process had helped interstitium’s scale remain elusive all this while

This technique, known as “Probe-based confocal laser endomicroscopy” or pCLE, allowed them to observe living human tissue rather than waiting for the fixed slide. Doing this showed them a net-like pattern in the bile-duct tissue that didn’t look like any anatomical discovery so far. The branches appeared to be 20µm wide. Upon further examination, the coauthors, along with Neil Theise, a pathologist at New York University’s School of Medicine, determined that the interstitium is made up of liquid-filled collagen bundles. Further samples cemented the presence of this structure in tissues in multiple areas of the body.

This wasn’t the first time in vivo microscopy had revealed a previously undiscovered anatomically significant part of the human body. In recent times, lymphatic vessels in the brain were identified for the first time using in vivo multiphoton microscopy imaging through a thinned skull preparation.

What does it mean for the human body?

Dr Michael Nathanson

Further examination from the researchers seems to indicate that there is a direct connection between the interstitial space and the lymphatic system. Apparently, the lymphatic fluid gets produced in the interstitium and flows into the lymphatic system. Hence, cancer tumours that get into this space can spread to lymph nodes. As the researchers have themselves put it, interstitium could be a veritable ‘water slide’ for cancer. If this turns out to be true, it will shed new light on how cancer spreads in our body.

This network was almost discovered in an older study. In 2011, Dr Michael Nathanson and colleagues from Yale University School of Medicine had published a paper highlighting their discovery of a network of dark fibres that they weren’t able to figure out properly.

There are other purposes that the interstitium could be fulfilling as well, like cushioning our organs, transporting white blood cells or molecules that are used to signal communication or more. It is the omnipresence of this network in our body that makes its potential capabilities so vast and diverse. But that brings us to the important question – is it an organ?

Is it an organ?

If you remember, about a year ago, popular media went into a similar tizzy over mesentery, which was – you guessed it – a new organ. A fan-shaped bunch of tissues that holds our gut in place, we had known about mesentery for a long while but only recently discovered it to be large enough to warrant a discussion over its organ status. Many pieces reported it as the body’s 79th organ. Truth be told, that number is entirely arbitrary.

That is because there is no central consensus over the definition of an organ. Biologists across the world do not have one particular way of telling if something in your body is an organ or not. In fact, they still debate about the topic with no conclusion in sight. The most generic definition that is thrown up with a simple Google search is ‘a part of an organism which is typically self-contained and has a specific vital function.’ Even a somewhat cursory search beyond this provides you with enough room to stretch the definition as you see fit. Which is why parts of our body like skin, eyeballs, nipples and the likes of mesenteries have been classified and declassified as organs quite arbitrarily over the years.

Mesentery, the deja vu to Interstitium

Is it really necessary to have the definition of an organ? You bet it is. As we increasingly integrate technology into healthcare and let artificial intelligence run havoc on medical data, we need to have clear and precise definitions of things. Algorithms running on medical data to identify newer ways of detecting diseases and ailments and finding novel ways to cure the same need to know that an organ in one place is an organ in another. A disparity, in this case, would make such algorithms, and consequently our efforts to integrate AI into healthcare, completely useless.

So where does that leave us?

At the moment? Exactly where we started off. Our classification of the interstitium as an organ (or the lack of it thereof) will, in no way, affect its actual functioning and importance. It will continue to fulfil its currently somewhat elusive purpose regardless of the scientific community’s ability to agree upon the definition of an organ.

Instead, what it does merit is further study into the literature that has acknowledged its existence in the past (there’s plenty of it) without classifying it as an organ to understand its true nature. In essence, we need to find out what we already know about it without knowing that we do. Confused? Well, maybe it will turn out that your interstitium can help you deal with it too!

Arnab Mukherjee

Arnab Mukherjee

A former tech-support desk jockey, you can find this individual delving deep into all things tech, fiction and food. Calling his sense of humour merely terrible would be a much better joke than what he usually makes.